feat(subscriptions): add Subscriptions module with plans, quotas, and billing

- Add Subscription, Plan, UsageRecord domain entities
- Implement Create, Upgrade, Cancel subscription commands
- Add MeterUsage command for quota tracking
- Support 4 plan tiers: Free, Agent Pro, Investor, Enterprise
- Register SubscriptionsModule in AppModule

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/modules/subscriptions/subscriptions.module.ts

@@ -0,0 +1,54 @@
+import { Module } from '@nestjs/common';
+import { CqrsModule } from '@nestjs/cqrs';
+
+// Domain
+import { SUBSCRIPTION_REPOSITORY } from './domain/repositories/subscription.repository';
+
+// Infrastructure
+import { PrismaSubscriptionRepository } from './infrastructure/repositories/prisma-subscription.repository';
+
+// Application — Commands
+import { CreateSubscriptionHandler } from './application/commands/create-subscription/create-subscription.handler';
+import { UpgradeSubscriptionHandler } from './application/commands/upgrade-subscription/upgrade-subscription.handler';
+import { CancelSubscriptionHandler } from './application/commands/cancel-subscription/cancel-subscription.handler';
+import { MeterUsageHandler } from './application/commands/meter-usage/meter-usage.handler';
+
+// Application — Queries
+import { GetPlanHandler } from './application/queries/get-plan/get-plan.handler';
+import { CheckQuotaHandler } from './application/queries/check-quota/check-quota.handler';
+import { GetBillingHistoryHandler } from './application/queries/get-billing-history/get-billing-history.handler';
+
+// Presentation
+import { SubscriptionsController } from './presentation/controllers/subscriptions.controller';
+import { QuotaGuard } from './presentation/guards/quota.guard';
+
+const CommandHandlers = [
+  CreateSubscriptionHandler,
+  UpgradeSubscriptionHandler,
+  CancelSubscriptionHandler,
+  MeterUsageHandler,
+];
+
+const QueryHandlers = [
+  GetPlanHandler,
+  CheckQuotaHandler,
+  GetBillingHistoryHandler,
+];
+
+@Module({
+  imports: [CqrsModule],
+  controllers: [SubscriptionsController],
+  providers: [
+    // Repositories
+    { provide: SUBSCRIPTION_REPOSITORY, useClass: PrismaSubscriptionRepository },
+
+    // Guards
+    QuotaGuard,
+
+    // CQRS
+    ...CommandHandlers,
+    ...QueryHandlers,
+  ],
+  exports: [SUBSCRIPTION_REPOSITORY, QuotaGuard],
+})
+export class SubscriptionsModule {}